Optical receiver signal strength indicator (RSSI) circuit having a variable supply voltage filter impedance

ABSTRACT

An optical receiver signal strength indicator (RSSI) circuit for use in an optical receiver or transceiver module is provided that uses a variable impedance device in the supply voltage filter circuit. The variable impedance device is varied based on the strength of the input current signal produced by the photodiode. At lower values of input current, the variable impedance is increased to improve the accuracy with which the RSSI circuit senses the input current, which improves the accuracy of the RSSI signal output from the RSSI circuit. The increase in impedance also improves supply voltage filtering by reducing the low-pass bandwidth of the supply voltage filter circuit. At higher values of input current, the variable impedance is decreased to ensure that the voltage bias applied to the photodiode is at least equal to a minimum bias voltage needed for proper operation of the photodiode.

TECHNICAL FIELD

The invention relates to optical communications modules. Moreparticularly, the invention relates to an optical receiver signalstrength indicator (RSSI) circuit for use in optical communicationsmodules.

BACKGROUND

A variety of optical communications modules exist for transmittingand/or receiving optical data signals over optical waveguides (e.g.,optical fibers). Optical communications modules include opticalreceiver, optical transmitter and optical transceiver modules. Opticalreceiver modules have one or more receive channels for receiving one ormore optical data signals over one or more respective opticalwaveguides. Optical transmitter modules have one or more transmitchannels for transmitting one or more optical data signals over one ormore respective optical waveguides. Optical transceiver modules have oneor more transmit channels and one or more receive channels fortransmitting and receiving respective optical transmit and receive datasignals over respective transmit and receive optical waveguides. Foreach of these different types of optical communications modules, avariety of designs and configurations exist.

In optical receiver and transceiver modules, an optical data signalpassing out of an end of an optical fiber is coupled by an optics systemonto an optical detector, such as a P-intrinsic-N (PIN) diode or othertype of photodiode. The photodiode converts the optical data signal intoan electrical current signal, which is then converted into an electricalvoltage signal, amplified and processed to recover the data. Thecurrent-to-voltage conversion and amplification processes are typicallyperformed by a transimpedance amplifier (TIA) circuit.

In many cases, it is desirable or necessary to provide an indicatorsignal that is indicative of the optical power level of the incidentlight striking the photodiode. The indicator signal is typicallyreferred to as a receiver signal strength indicator (RSSI) signal, andthe signal may be either an analog or digital signal and may or may notbe amplified. Known RSSI circuits exist for determining the opticalpower level of the incident light based on a measurement of theelectrical current produced by the photodiode.

A typical RSSI circuit includes a filter circuit for filtering out highfrequency noise applied to the photodiode by the supply voltage. Thefilter circuit typically includes a resistor and a capacitor connectedin series. By sensing the voltage across the resistor, the input currentsignal output by the photodiode to the RSSI circuit is sensed. The inputcurrent signal is proportional to the input optical power, i.e., theoptical power level of the light striking the photodiode. Hence, theRSSI circuit detects the input optical power.

FIG. 1 illustrates a block diagram of a typical RSSI circuit 1 forgenerating an electrical current signal proportional to the photocurrentproduced by a photodiode 2 when light strikes the photodiode 2. Thecathode of the photodiode 2 is connected to a supply voltage filtercircuit comprising a first resistor, R1, 3 and a capacitor, C_(FLT), 4.The anode of the photodiode 2 is connected to an input of a TIA 6. Thesupply voltage filter circuit acts as a low-pass filter that removeshigh frequency noise from the supply voltage, V_(CC). The input current,I_(PIN), produced by the photodiode 2 flows through resistor R1 3, whichgenerates a time-varying voltage signal that is dependent on I_(PIN).Because the RC time constant associated with R1 and C_(FLT) is muchlarger than the data rate of the RSSI circuit 1, the voltage, V1, acrossR1 varies very little with time and is therefore useful in tracking theaverage input current, which is calculated as V1/R1.

An operational amplifier (op-amp) 5, a second resistor, R2, 7 and ap-type metal oxide semiconductor transistor (PMOS) 8 are used togenerate an output current, I_(OUT), proportional to the average inputcurrent I_(PIN)*(R1/R2), where the symbol “*” represents amultiplication operation. The RSSI circuit 1 will force the same voltageV1 that is across R1 to be across R2, creating a current in R2 equal toV1/R2 that flows in and out of the PMOS 8 and into an appropriate load 9having a load impedance, Z_(LOAD). The output current, I_(OUT), whichequals I_(PIN)*(R1/R2), is normally considered the RSSI signal and thissignal is typically used by other circuitry (not shown) to monitor theoptical power level of the photodiode 2. In some cases, the RSSI signalis amplified and/or digitized.

With RSSI circuits having the configuration shown in FIG. 1, the voltagebias from the supply voltage V_(CC) that is applied to the photodiode 2must be kept above a minimum value in order for the photodiode 2 tooperate properly. This minimum voltage value for the photodiode 2 limitsthe allowed voltage drop V1 across R1, which constrains the value of R1to a small value. If the value of R1 is too large, the voltage drop V1will not be sufficiently below V_(CC) to ensure that the voltage biasapplied to the photodiode 2 is large enough for it to operate properly.Therefore, when using the RSSI circuit configuration shown in FIG. 1,the value of R1 must be chosen so that it is small enough to supportmaximum current flow and to maintain a voltage level across R1 that issufficiently below V_(CC). On the other hand, a large value for R1 isdesired in order to reduce the low-pass bandwidth of the filter circuitand to provide a signal that is sufficiently large to allow accuratesensing of the input current signal.

Accordingly, a need exists for an RSSI circuit that ensures that theimpedance of the supply voltage filter circuit is small enough that thephotodiode has adequate voltage to operate properly and large enough toensure accurate sensing of the input current signal and effective supplyvoltage filtering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a known RSSI circuit forgenerating an RSSI signal proportional to a photocurrent produced by aphotodiode.

FIG. 2 illustrates a block diagram of an RSSI circuit in accordance withan illustrative embodiment.

FIG. 3 illustrates a block diagram of an RSSI circuit in accordance withanother illustrative embodiment.

FIG. 4 illustrates a block diagram of an RSSI circuit in accordance withanother illustrative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In accordance with illustrative, or exemplary, embodiments describedherein, an RSSI circuit for use in an optical receiver or transceivermodule is provided that uses a variable impedance device in the supplyvoltage filter circuit. The variable impedance device has an impedancevalue that is varied based directly or indirectly on the strength of theinput current signal produced by the photodiode. At lower values ofinput current (signal is weak), the variable impedance is increased toimprove the accuracy with which the RSSI circuit senses the inputcurrent, which improves the accuracy of the RSSI signal output from theRSSI circuit. The increase in impedance also improves supply voltagefiltering by reducing the low-pass bandwidth of the supply voltagefilter circuit. At higher values of input current (signal is strong),the variable impedance is decreased to ensure that the voltage biasapplied to the photodiode is at least equal to a minimum bias voltageneeded for proper operation of the photodiode. Illustrative, orexemplary, embodiments of the RSSI circuit will now be described withreference to FIGS. 2-4, in which like reference numerals represent likecomponents, elements or features.

FIG. 2 illustrates a block diagram of an RSSI circuit 10 in accordancewith an illustrative embodiment that includes a variable impedancedevice having an impedance value that is varied based on the value ofthe RSSI signal produced by the RSSI circuit 10. The RSSI circuit 10includes a supply voltage filter circuit that comprises a variableimpedance device 11 and a filter capacitor, C_(FLT), 12. The anode of aphotodiode 13 is electrically coupled to an input of a TIA 15 and thecathode of the photodiode 13 is electrically coupled to the variableimpedance device 11 and to an input of an amplifier 14. The TIA 15 is aconventional circuit of an optical receiver and therefore, in theinterest of brevity, will not be described herein. The input currentI_(PIN) from the photodiode 13 flows through the variable impedancedevice 11 and causes a voltage across the variable impedance device 11to be generated. The difference between this voltage and V_(CC) is thenamplified by amplifier 14 by a gain of K/Z, where K is a numericalconstant and Z is the present impedance value of the variable impedancedevice 11. The output of the amplifier 14 is the RSSI signal, which isproportional to I_(PIN). The RSSI signal is provided to the variableimpedance device 11. For higher values of I_(PIN), the RSSI signal valueis also higher, which causes the impedance value of the variableimpedance device 11 to be reduced. For lower values of I_(PIN), the RSSIsignal value is also lower, which causes the impedance value of thevariable impedance device 11 to be increased.

FIG. 3 illustrates an RSSI circuit 30 in accordance with anotherillustrative embodiment. The RSSI circuit 30 includes a supply voltagefilter circuit that comprises a capacitor C_(FLT) 31, resistors R1 32and R3 33, and a first PMOS M1 35. A control circuit 34 controls theactivation and deactivation of the first PMOS M1 35 and of a second PMOSM2 36. The network comprising PMOS M2 36 and resistors R2 37 and R4 38is a scaled replica of the network comprising PMOS M1 35 and resistorsR1 32 and R3 33. In this RSSI circuit 30, R1/R2=R3/R4=(width/length(W/L) of M2)/(W/L of M1)=N, and therefore the overall impedance of thereplica network is scalable by a factor of 1/N. The op-amp 41 and thePMOS 42 force the same voltage drop across both of the networks. Hence,the resulting output current signal, I_(OUT), will be equal toI_(PIN)*N. The current that passes into and out of the PMOS 42 isdelivered to a load 43, Z_(LOAD). The output current signalI_(OUT)=I_(PIN)*N is the RSSI signal produced by the RSSI circuit 30.

The variable impedance device of the RSSI circuit 30 shown in FIG. 3comprises resistors R1 32, R3 33 and PMOS M1 35. The control signal thatis applied to control circuit 34 activates or deactivates the PMOSs M135 and M2 36, depending on the value of the control signal. Inaccordance with an illustrative embodiment, the control signal is basedon the value of the voltage signal on line 47 at the positive, ornon-inverting, terminal of the op-amp 41. The control signal may insteadbe based on, for example, the value of the voltage signal on line 46 atthe negative, or inverting, terminal of the op-amp 41, the RSSI signalitself (I_(PIN)*N), or the data signal output from the TIA 48. The anodeof a photodiode 45 is electrically coupled to an input of the TIA 48 andthe cathode of the photodiode 45 is electrically coupled to the resistorR1 32 and to the non-inverting terminal of op-amp 41. Because the TIA 48is a conventional circuit of an optical receiver, it will not bedescribed herein in the interest of brevity.

When the photodiode 45 is detecting low optical input powers, thecontrol signal will have a low value, which will cause the gate voltagesof the PMOSs M1 35 and M2 36 to be large, thereby forcing the PMOSs M135 and M2 36 to have relatively high impedances, or be in their “off”states. When the PIN diode 45 is detecting high optical input powers,the control signal will have a high value, which forces the PMOSs M1 35and M2 36 to have relatively low impedances, or be in their “On” states.The different voltages on the gate of PMOS M1 35 provide differentparallel impedances to resistor R3 33 that change the effectiveimpedance of the network of devices connecting the cathode of photodiode45 to the RSSI supply voltage, V_(CC). In this way, the impedance of thesupply voltage filter circuit is varied based on the value of the RSSIsignal.

It should be noted that the RSSI circuit 30 could operate without thefixed resistor R1 32. The variable impedance provided by the parallelarrangement of resistor R3 33 and PMOS M1 35 is able to achieve thegoals described above without resistor R1 32 if resistor R3 33 and PMOSM1 35 are suitably selected. However, because the resistor R1 32 can beproduced with a smaller parasitic capacitance than most devices,including R1 32 in the RSSI circuit 30 ensures higher filteringeffectiveness at higher frequencies. The network comprising resistors R237 and R4 38 and PMOS M2 36 should match the network comprisingresistors R1 32 and R3 33 and PMOS M1 35. Therefore, if resistor R1 32is eliminated, resistor R2 37 should also be eliminated.

In an experiment conducted with an RSSI circuit having the configurationshown in FIG. 3, a value for resistor R3 33 of 5000 ohms and a value forresistor R1 32 of 100 ohms were chosen. At an average input current of1.0 μA, PMOS M1 35 was in the Off state and the resulting voltage acrossthe networks was 5.1 mV, which is a significant improvement over the 0.1mV voltage across the network for the known RSSI circuit described abovewith reference to FIG. 1 when using similar resistor values for R1. Athigher input currents, PMOS s M1 35 and M2 36 will have very low gatevoltages and therefore very low impedances such that the overall networkimpedance will be only slightly larger than 100 ohms. If 100 ohms athigh power is mandatory, the resistors and transistors of the circuit 30can be easily scaled to meet the requirement.

FIG. 4 illustrates an RSSI circuit 50 in accordance with anotherillustrative embodiment. The RSSI circuit 50 is identical to the RSSIcircuit 30 shown in FIG. 3 except that the resistors R3 33 and R4 38shown in FIG. 3 have been replaced by first and second Schottky diodesD1 51 and D2 52, respectively, in FIG. 4. The supply voltage filtercircuit of the RSSI circuit 50 comprises capacitor C_(FLT) 31, resistorsR1 32, diode D1 51, and a first PMOS M1 35. The control circuit 34controls the activation and deactivation of the first and second PMOSsM1 35 and M2 36. The network comprising PMOS M2 36, resistor R2 37 anddiode D2 52 is a scaled replica of the network comprising PMOS M1 35 andresistor R1 32 and diode D1 51. In the RSSI circuit 50, R1/R2=(Area ofD2)/(Area of D1)=(W/L of M2)/(W/L of M1)=N, and therefore the overallimpedance of the replica network is scalable by a factor of 1/N. Theop-amp 41 and the PMOS 42 force the same voltage drop across both of thenetworks. Hence, the resulting output current signal, I_(OUT), will beequal to I_(PIN)*N. The current that passes into and out of the PMOS 42is delivered to a load 43, Z_(LOAD). The output current signalI_(OUT)=I_(PIN)*N is the RSSI signal produced by the RSSI circuit 50.

The variable impedance device of the RSSI circuit 50 shown in FIG. 4comprises resistor R1 32, diode D1 51 and PMOS M1 35. The control signalthat is applied to control circuit 34 activates or deactivates the PMOSsM1 35 and M2 36, depending on the control signal value. In accordancewith an illustrative embodiment, the control signal is based on thevalue of the voltage signal on line 47 at the positive terminal of theop-amp 41. The control signal may instead be based on, for example, thevalue of the voltage signal on line 46 at the negative terminal of theop-amp 41, the RSSI signal itself (I_(PIN)*N), or the data signal outputfrom the TIA circuit (not shown) of the receiver or transceiver modulethat incorporates the RSSI circuit 50.

When the photodiode 45 is detecting low optical input powers, thecontrol signal will have a low value, which will cause the gate voltagesof the PMOSs M1 35 and M2 36 to be large, thereby forcing the PMOSs M135 and M2 36 into high impedance states. When the PIN diode 45 isdetecting high optical input powers, the control signal will have a highvalue, which forces the PMOSs M1 35 and M2 36 into low impedance states.In addition, the diodes D1 51 and D2 52 have impedances that scale downnaturally with increased current and that scale up naturally withdecreased current. In this way, the impedance of the supply voltagefilter circuit is varied based on the value of the RSSI signal. Becauseof the manner in which the diodes D1 51 and D2 52 naturally change theirimpedance values as the current changes, the RSSI circuit 50 can operateeffectively in certain cases without the PMOSs M1 35, M2 36 and thecontrol circuit 34. Therefore, in some embodiments, the PMOSs M1 35 andM2 36 and the control circuit 34 are eliminated.

It should be noted that the invention has been described with respect toillustrative embodiments for the purposes of demonstrating theprinciples and concepts of the invention. The invention is not limitedto these embodiments, as will be understood by persons of skill in theart. For example, while the invention has been described with referenceto particular supply voltage filtering circuits having variableimpedances, the principles and concepts of the invention can be achievedusing a variety of RSSI circuit configurations, as will be understood bythose skilled in the art in view of the description being providedherein. Many modifications may be made to the embodiments describedherein while still achieving the goals of the invention, and all suchmodifications are within the scope of the invention.

What is claimed is:
 1. An optical receiver signal strength indicator(RSSI) circuit for use in an optical receiver or transceiver module, theRSSI circuit comprising: a supply voltage filter circuit beingconfigured to act as a filter that removes noise of a predeterminedfrequency range from a supply voltage of the RSSI circuit, the supplyvoltage filter circuit being electrically coupled to the supply voltageof the RSSI circuit, the supply voltage filter circuit comprising afirst variable impedance device, wherein an impedance value of thevariable impedance device is increased when an input current signal tothe RSSI circuit is weak and is decreased when the input current signalis strong, the input current signal being produced by an opticaldetector that is electrically coupled to an input terminal of the RSSIcircuit, the input current signal being a current signal produced by theoptical detector in response to light striking the optical detector. 2.The RSSI circuit of claim 1, wherein the supply voltage filter circuitfurther comprises: at least a first resistor having a fixed value and afirst capacitor having a capacitance, the first resistor and the firstcapacitor being electrically coupled to one another and to the inputterminal of the RSSI circuit, the first resistor being electricallycoupled to the variable impedance device.
 3. The RSSI circuit of claim2, further comprising: an operational amplifier (op-amp) having anon-inverting terminal, an inverting terminal, and an output terminal,the non-inverting terminal being electrically coupled to the inputterminal of the RSSI circuit.
 4. The RSSI circuit of claim 3, furthercomprising: a second resistor electrically coupled to the invertingterminal of the op-amp; a second variable impedance device electricallycoupled to the supply voltage and to the second resistor, the secondvariable impedance device being a scaled replica of the first variableimpedance device; and a first transistor having a first terminal, asecond terminal and a third terminal, the first terminal of the firsttransistor being electrically coupled to the output terminal of theop-amp, the second terminal of the first transistor being electricallycoupled to the second resistor and to the inverting terminal of theop-amp.
 5. The RSSI circuit of claim 4, wherein the first variableimpedance device comprises a third resistor and a second transistor thatare connected in parallel to one another, and wherein the secondvariable impedance device comprises a fourth resistor and a thirdtransistor that are connected in parallel to one another, the RSSIcircuit further comprising: control circuitry electrically coupled tothe second and third transistors, the control circuitry receiving acontrol signal at an input terminal thereof and outputting a signal atan output terminal thereof that controls respective impedances of thesecond and third transistors to vary the respective impedances of thefirst and second variable impedance devices.
 6. The RSSI circuit ofclaim 5, wherein the first, second and third transistors are P-typeMetal Oxide Semiconductor Transistors (PMOSs), and wherein the controlcircuitry comprises an inverting control circuit that activates thesecond and third transistors when the control signal is large and thatdeactivates the second and third transistors when the control signal issmall, wherein when the second and third transistors are deactivated,the second and third transistors are in relatively high-impedance statesand wherein when the second and third transistors are activated, thesecond and third transistors are in relatively low-impedance states. 7.The RSSI circuit of claim 5, wherein the control signal is based on orderived from a voltage signal applied to the non-inverting terminal ofthe op-amp.
 8. The RSSI circuit of claim 5, wherein the control signalis based on or derived from a voltage signal applied to the invertingterminal of the op-amp.
 9. The RSSI circuit of claim 5, wherein thecontrol signal is based on or derived from an RSSI output signal of theRSSI circuit.
 10. The RSSI circuit of claim 5, wherein the controlsignal is based on or derived from other circuitry of the opticalreceiver or transceiver module that receives the current signal producedby the optical detector.
 11. The RSSI circuit of claim 4, wherein thefirst variable impedance device comprises a first diode having an anodethat is electrically coupled to the supply voltage and having a cathodethat is electrically coupled to the first resistor, and wherein thesecond variable impedance device comprises a second diode having ananode that is electrically coupled to the supply voltage and having acathode that is electrically coupled to the second resistor, and whereinthe first and second diodes have respective impedances that increase asthe input current signal decreases and that decrease as the inputcurrent signal increases.
 12. The RSSI circuit of claim 11, wherein thefirst variable impedance device further comprises a second transistorconnected in parallel with the first diode, and wherein the secondvariable impedance device comprises a third transistor connected inparallel with the second diode, the RSSI circuit further comprising:control circuitry electrically coupled to the second and thirdtransistors, the control circuitry receiving a control signal at aninput terminal thereof and outputting a signal that activates ordeactivates the second and third transistors to vary the respectiveimpedances of the first and second variable impedance devices.
 13. TheRSSI circuit of claim 12, wherein the first, second and thirdtransistors are P-type Metal Oxide Semiconductor Transistors (PMOSs),and wherein the control circuitry comprises an inverting control circuitthat activates the second and third transistors when the control signalis large and that deactivates the second and third transistors when thecontrol signal is small, wherein when the second and third transistorsare deactivated, the second and third transistors are in relativelyhigh-impedance states and wherein when the second and third transistorsare activated, the second and third transistors are in relativelylow-impedance states.
 14. The RSSI circuit of claim 12, wherein thecontrol signal is based on or derived from a voltage signal applied tothe non-inverting terminal of the op-amp.
 15. The RSSI circuit of claim12, wherein the control signal is based on or derived from a voltagesignal applied to the inverting terminal of the op-amp.
 16. The RSSIcircuit of claim 12, wherein the control signal is based on or derivedfrom an RSSI output signal of the RSSI circuit.
 17. The RSSI circuit ofclaim 12, wherein the control signal is based on or derived from othercircuitry of the optical receiver or transceiver module that receivesthe current signal produced by the optical detector.
 18. An opticalreceiver signal strength indicator (RSSI) circuit for use in an opticalreceiver or transceiver module, the RSSI circuit comprising: a supplyvoltage filter circuit electrically coupled to a supply voltage of theRSSI circuit and to an input terminal of the RSSI circuit, the supplyvoltage filter circuit comprising a first variable impedance device; asecond variable impedance device that is a scaled replica of the firstvariable impedance device, the second variable impedance device beingelectrically coupled to the supply voltage; an operational amplifier(op-amp) having a non-inverting terminal, an inverting terminal, and anoutput terminal, the non-inverting terminal being electrically coupledto the input terminal of the RSSI circuit, the inverting terminal of theop-amp being electrically coupled to the second variable impedancedevice; and control circuitry electrically coupled to the first andsecond variable impedance devices, the control circuitry beingconfigured to selectively vary an impedance value of at least the firstvariable impedance device based on a strength or weakness of an inputcurrent signal to the RSSI circuit, the input current signal beingproduced by an optical detector that is electrically coupled to theinput terminal of the RSSI circuit.
 19. The RSSI circuit of claim 18,wherein the control circuitry increases the impedance value of the firstvariable impedance device when an input current signal to the RSSIcircuit is weak and decreases the impedance value of the first variableimpedance device when the input current signal to the RSSI circuit isstrong.
 20. A method for use in an optical receiver signal strengthindicator (RSSI) circuit of an optical receiver or transceiver module,the method comprising: detecting a strength or weakness of an inputcurrent signal received at an input terminal of the RSSI circuit, theinput current corresponding to a current signal produced by an opticaldetector that is electrically coupled to the input terminal; and basedon the detected strength or weakness of the input current signal,varying an impedance value of a variable impedance device of a supplyvoltage filter circuit of the RSSI circuit, the supply voltage filtercircuit acting as a filter that removes noise of a predeterminedfrequency range from a supply voltage of the RSSI circuit, the supplyvoltage filter circuit being electrically coupled to the supply voltageof the RSSI circuit.
 21. The method of claim 20, wherein the impedancevalue of the variable impedance device is increased when the detectedstrength of the input current signal is weak and is decreased when thedetected strength of the input current signal is strong.